The Ebola infected US aid workers, Dr. Kent Brantly and Nurse Nancy Writebol, appear to be improving after receiving an experimental drug, officials have said.

It is not clear if the ZMapp drug, which has only been tested on monkeys, can be credited with their improvement.

Dr. Kent Brantly was flown from Liberia to Atlanta for treatment on Saturday. His colleague Nancy Writebol arrived back in the city of Atlanta on Tuesday.

Since February, 887 people have died of Ebola in four West African countries.

The World Bank is allocating $200 million in emergency assistance for countries battling to contain the Ebola outbreak.

Dr. Kent Brantly and Nurse Nancy Writebol’s condition appear to be improving after receiving Ebola experimental serum ZMapp

It is the world’s deadliest outbreak to date and has centered on Guinea, Liberia and Sierra Leone. There have also been two cases in the Nigerian city of Lagos, where eight people are currently in quarantine.

British Airways has temporarily suspended flights to and from Liberia and Sierra Leone until August 31, 2014, because of the health crisis, the airline said in a statement. It follows a similar suspension by two regional air carriers last week.

The Ebola virus spreads by contact with infected blood and bodily fluids. The current outbreak is killing between 50% and 60% of people infected.

There is no cure or vaccine for Ebola – but patients have a better chance of survival if they receive early treatment.

Dr. Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, says clinical trials are to start in September on an Ebola vaccine that has shown promising results during tests on animals.

Dr. Kent Brantly and Nancy Writebol were treated with the ZMapp serum before their evacuation from Liberia.

According to a CNN report, quoting a doctor in Liberia, Dr. Kent Brantly’s condition improved dramatically within an hour of receiving the drug.

Service in Mission (SIM), the Christian aid group that employs Nancy Writebol, says she has had two doses of the drug and did not respond as well as Dr. Kent Brantly but she is showing “improvement”.

“She is walking with assistance… strength is better… has an appetite,” SIM spokesman Palmer Holt told the Washington Post newspaper in an email on Monday.

Nancy Writebol is on her way to a special isolation ward at Atlanta’s Emory University Hospital, where Dr. Kent Brantly is being treated by infectious disease specialists.